In stirred-tank bioreactors used for laboratory-scale or pilot-scale mammalian cell culture, foam generation due to the injection of gas containing oxygen in the reactor is one of the more critical problems which remain to be solved: in fact, accumulation of foam at the culture medium surface decreases the gas transfer rate (oxygen from air to culture medium and CO.sub.2 from culture medium to air), which is a relevant parameter for a correct and continuous growth of the mammalian cells.
If the gas transfer due to surface aeration is unable to supply the whole amount of oxygen required for the cell growth, in some cases direct sparging of air into the culture medium is used, but said direct sparging of air involves foam production which in turn reduces the surface air/liquid gas transfer rate: it is known that the increased oxygen flow rate required to maintain the desired oxygen concentration in the bioreactor culture medium has (or could have) detrimental effects on cell viability.
The production of foam due to gas sparging is particularly detrimental if said mammalian cells are grown on microcarriers as the air sparging bubbles carry along the microcarriers making them float at the culture medium surface where they remain trapped on a foam bubble network: this phenomena leads to a decrease of the amount of microcarriers and of the cell density in the reactor and consequently so a decrease of the overall reactor productivity.
Formation of very large cell and microcarrier aggregates can lead to non-homogeneous culture and makes cleaning of the bioreactor more difficult at the end of a run.
The solution presently used to solve the foam problem provides that an anti-foam chemical product (normally a silicone emulsion made, for example, of 30% Simethicone USP plus 14% stearate emulsifier and 0.075% sorbic acid in water) is added to the liquid, but such a solution creates several problems, may have some toxic effects on some cell lines and reduce recombinant protein expression by genetically engineered cells.
Over the last years the use of a mechanical surface aerator had been investigated as a different approach to the problem of reaching a proper surface aeration without gas sparging (or at least with a reduced gas sparging), so avoiding or at least limiting the production of foam.
W. S. HU and al. (Biotechnol. Bioeng. Vol. XXVIII, pp 122-125, 1986) disclose a mechanical surface aerator mounted on the same shaft of the impeller to improve gas transfer from the gas/liquid interface in the liquid.
Said surface aerator is particularly suitable if applied to small size laboratory bioreactors and increases turbulence of the liquid surface so improving surface aeration and reducing the need of gas sparging: as a consequence, a reduced foam production occurs.
As the same Authors say, one use of a surface aerator will surely not solve oxygen transfer problems in large scale operations; however, on any scale in which surface aeration contributes a significant extent of the oxygen transferred to the culture, a surface aerator will certainly have an enhancement effect.
EP-A-0 257 750 refers to a bioreactor equipped with a surface aerator made of a metallic screen and moved by suitable moving means: said screen is on a float allowing it to stay at or just below the surface of the liquid.
The surface of the screen is parallel to that of the liquid and its rotation generates turbulence at the surface of the liquid, thus improving the oxygen transfer rates.
The Applicant discovered that when a perforated plate made of an hydrophobic material rotates at the surface of a liquid filled in a bioreactor in a perpendicular position (or with an inclination from 45.degree. to 90.degree.) in respect to said surface and partially immersed into said liquid, a surprising foam breaking effect occurs. Moreover, said anti-foam device allows a significant improvement of the surface aeration rate of the liquid by increasing the gas exchange rate at the gas/liquid interface.
On a preferred embodiment, said hydrophobic perforated plate is held by a stainless steel support.